1. Photosynthesis: Life from Light

2. How are they connected? Heterotrophs and Autotrophs  Autotrophs 
making energy & organic molecules from ingesting organic molecules
glucose + oxygen  carbon + water + energy
dioxide
C6H12O6
6O2
6CO2
6H2O
ATP  +
exergonic
Where’s the ATP?
Autotrophs
So, in effect, photosynthesis is respiration run backwards powered by light.
Cellular Respiration
oxidize C6H12O6  CO2 & produce H2O
fall of electrons downhill to O2
exergonic
Photosynthesis
reduce CO2  C6H12O6 & produce O2
boost electrons uphill by splitting H2O
endergonic
making energy & organic molecules from light energy
+ water + energy  glucose + oxygen
carbon
dioxide
6CO2
6H2O
C6H12O6
6O2
light
energy  +
endergonic

3. Plant structure Obtaining raw materials sunlight CO2 H2O Nutrients
leaves = solar collectors
CO2
stomates = gas exchange
Found under leaves
H2O
uptake from roots
Nutrients
N, P, K, S, Mg, Fe…

4. Plant structure Chloroplasts Chlorophyll & ETC in thylakoid membrane
double membrane
stroma
thylakoid sacs
grana stacks
Chlorophyll & ETC in thylakoid membrane
H+ gradient built up within thylakoid sac
A typical mesophyll cell has chloroplasts, each about 2-4 microns by 4-7 microns long.
Each chloroplast has two membranes around a central aqueous space, the stroma.
In the stroma are membranous sacs, the thylakoids.
These have an internal aqueous space, the thylakoid lumen or thylakoid space.
Thylakoids may be stacked into columns called grana. H+

5. Pigments of photosynthesis
Why does this structure make sense?
chlorophyll & accessory pigments
“photosystem”
embedded in thylakoid membrane
structure  function
Orientation of chlorophyll molecule is due to polarity of membrane.

6. Light: absorption spectra
Photosynthesis gets energy by absorbing wavelengths of light
chlorophyll a (dominant pigment)
absorbs best in red & blue wavelengths & least in green
other pigments with different structures absorb light of different wavelengths
Why are plants green?

7. It’s the Dark Reactions!
Photosynthesis
Light reactions
light-dependent reactions
energy production reactions
convert solar energy to chemical energy
ATP & NADPH
Calvin cycle
light-independent reactions
sugar production reactions
uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6
It’s the Dark Reactions!

8. Light reactions Electron Transport Chain (like cell respiration!)
membrane-bound proteins in organelle
electron acceptor
NADPH
proton (H+) gradient across inner membrane
ATP synthase enzyme
Not accidental that these 2 systems are similar, because both derived from the same primitive ancestor.

9. Photosystems 2 photosystems in thylakoid membrane reaction center
act as light-gathering “antenna complex”
Photosystem II
chlorophyll a
P680 = absorbs 680nm wavelength red light
Photosystem I
chlorophyll b
P700 = absorbs 700nm wavelength red light
reaction center
Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane.
When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center.
At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a.
This starts the light reactions.
Don’t compete with each other, work synergistically using different wavelengths.

10. ETC of Photosynthesis ETC produces from light energy
ATP & NADPH
NADPH (stored energy) goes to Calvin cycle
PS II absorbs light
excited electron passes from chlorophyll to “primary electron acceptor” at the REACTION CENTER.
splits H2O (Photolysis!!)
O2 released to atmosphere
ATP is produced for later use

11. ETC of Photosynthesis Photosystem II Photosystem I
Two places where light comes in.
Remember photosynthesis is endergonic -- the electron transport chain is driven by light energy.
Need to look at that in more detail on next slide

12. Noncyclic Photophosphorylation
Light reactions elevate electrons in 2 steps (PS II & PS I)
PS II generates energy as ATP
PS I generates reducing power as NADPH
1 photosystem is not enough.
Have to lift electron in 2 stages to a higher energy level. Does work as it falls.
First, produce ATP -- but producing ATP is not enough.
Second, need to produce organic molecules for other uses & also need to produce a stable storage molecule for a rainy day (sugars). This is done in Calvin Cycle!

13. Cyclic photophosphorylation
If PS I can’t pass electron to NADP, it cycles back to PS II & makes more ATP, but no NADPH
coordinates light reactions to Calvin cycle
Calvin cycle uses more ATP than NADPH X

14. From Light reactions to Calvin cycle
Chloroplast stroma
Need products of light reactions to drive synthesis reactions
ATP
NADPH
What is there
left to do?
Make sugar!

15. From CO2  C6H12O6 CO2 has very little chemical energy
fully oxidized
C6H12O6 contains a lot of chemical energy
reduced
endergonic
Reduction of CO2  C6H12O6 proceeds in many small uphill steps
each catalyzed by specific enzyme
using energy stored in ATP & NADPH

16. Calvin cycle 1C 5C 6C 3C 3C 3C 2x x2 2x CO2 1. Carbon fixation
ribulose
bisphosphate
1. Carbon fixation
3. Regeneration
of RuBP 5C
RuBP
Rubisco 6C
3 ADP
3 ATP
-enzyme that
Binds CO2
to RuBP
PGAL
to make
glucose 3C 2x
PGA 3C x2
PGAL
sucrose
cellulose
etc.
RuBP = ribulose bisphosphate
Rubisco = ribulose bisphosphate carboxylase
PGA = phosphoglycerate
PGAL = phosphoglyceraldehyde
2. Reduction
6 NADP
6 NADPH
6 ADP
6 ATP 3C 2x

17. Calvin cycle PGAL   important intermediate
Six turns of Calvin Cycle = 1 glucose
PGAL   glucose   carbohydrates
  lipids
  amino acids
  nucleic acids

18. Summary Light reactions Calvin cycle produced ATP produced NADPH
consumed H2O
produced O2 as by product
Calvin cycle
consumed CO2
produced PGAL
regenerated ADP
regenerated NADP
ADP
NADP

19. Factors that affect Photosynthesis
Enzymes are responsible for several photosynthetic processes, therefore, temperature and pH can affect the rate of photosynthesis.
The amount and type of light can affect the rate.
A shortage of any of the reactants,CO2 and/or H2O, can affect the rate.

20. Supporting a biosphere
On global scale, photosynthesis is the most important process for the continuation of life on Earth
each year photosynthesis synthesizes 160 billion tons of carbohydrate
heterotrophs are dependent on plants as food source for fuel & raw materials

21. The Great Circle of Life!
Energy cycle
sun
Photosynthesis
glucose O2
H2O
CO2
Cellular Respiration
The Great Circle of Life!
Where’s Mufasa?
ATP

22. Summary of photosynthesis
6CO2
6H2O
C6H12O6
6O2
light
energy  +
Where did the CO2 come from?
Where did the CO2 go?
Where did the H2O come from?
Where did the H2O go?
Where did the energy come from?
What’s the energy used for?
What will the C6H12O6 be used for?
Where did the O2 come from?
Where will the O2 go?
What else is involved that is not listed in this equation?

23. Alternative Pathways
The Calvin Cycle is the MOST Common Pathway for Carbon Fixation. Plant Species that fix Carbon EXCLUSIVELY through the Calvin Cycle are known as C3 PLANTS. 
Plants in hot dry environments have a problem with water loss, so they keep their stomata partly closed... this results in:
CO2  deficit (Used in Calvin Cycle), and the level of O2 RISES (as Light reactions Split Water Molecules).

24. Figure 7.10
C4 plants and CAM plants use an alternate pathway to FIX carbon dioxide from the air.
Figure 7.10

25. Figure 7.11
THE CAM PATHWAY - Plants that use the CAM Pathway open their stomata at night and close during the day.
At night, CAM Plants take in CO2 and fix into organic compounds. During the day, CO2 is released from these Compounds and enters the Calvin Cycle. Because they have their stomata open only at night, they grow slow.
Figure 7.11